1. Field of the Invention
The present disclosure relates to a ramp signal generator and an image sensor.
2. Description of the Related Art
An image sensor is a device for capturing an image by using the property of semiconductors that react to light, and recently, the development of a CMOS (Complementary Metal-Oxide-Semiconductor) technique has promoted the common use of a CMOS image sensor.
The CMOS image sensor, which employs a CDS (Correlated Double Sampling) scheme, outputs the difference between a reset signal sampled according to the CDS scheme, and an image signal, as a digital signal.
Here, in order to output the difference between the reset signal and the image signal, as a digital signal, a ramp signal is used. For example, the CMOS image sensor picks up the difference between the image signal that varies according to the intensity of illumination of external light and the reset signal, converts the difference into a digital signal according to the slope of the ramp signal, and outputs the same.
In this case, on the assumption that the slope of the ramp signal is constant, when a portion (a brighter portion) in which the difference between the image signal and the reset signal is large and a portion (a darker portion) in which the difference between the image signal and the reset signal is small coexist, if the slope of the ramp signal is steep, it would be difficult to read a fine difference between pixels in the darker portion, making it impossible to discriminate the pixels. Conversely, if the slope of the ramp signal was gentle, pixels at the brighter portion could not be discriminated.